1. Field of the Invention
The present invention relates to a detection method and apparatus; in particular, to a method and apparatus for detecting the state of charge (SOC) of a battery.
2. Description of Related Art
In order to ensure safe operations of a battery, over-charge or over-discharge should be prevented to protect the battery from irreversible change of electrochemical characteristic inside the battery which deteriorates the performance and lowers the lifespan of the battery. Therefore, in order to avoid over-charge or over-discharge, the processes of charging or discharging must be stopped before the battery reaches its upper limit of charging capacity or the lower limit of discharging capacity. Consequently, by measuring the state of charge (SOC) of the battery, the change of the electrochemical characteristic of the battery could be monitored during battery charge/discharge processes to ensure that the battery is being used in a safe range.
However, the state of charge of a battery is affected by numerous factors, such as the history of charge/discharge operations, types of the battery (using different active materials), or the internal architecture of the battery. Hence, several methods measuring the SOC have been proposed in which the SOC of the battery may be obtained by means of detecting parameters varying along with the changes in the SOC of the battery. Those methods include: discharge test, electrolyte concentration measuring, coulomb/ampere hour counting, loaded voltage measuring, internal resistance measuring, open circuit voltage measuring, electrochemical impedance spectroscopy, and so forth.
However, the conventional measuring methods are associated with their disadvantages. For example, the open circuit voltage measuring method firstly open-circuits the battery and awaits the battery to its equivalent state (depolarized state), in which the relationship between the SOC of the battery and the open-circuit voltage is linear in a certain range (operation range), as shown in FIG. 1. According to FIG. 1, within an operating range S1 where the linear relationship exists for the SOC of the battery and the open-circuit voltage, the current SOC may be obtained by measuring the open-circuit voltage. Although the open circuit voltage measuring is relatively simpler, the measuring may take too much time in remaining the battery stationary to reach an equivalent state. Also, the open-circuit voltage method may be not accurate for those batteries whose open-circuit voltage changes comparatively small with the SOC (e.g., a lithium iron phosphate battery).
In addition, the electrochemical impedance spectroscopy measuring method is to provide multiple sets of waveforms of different frequencies to the battery and read the responded waveforms from the battery for analysis, and then parameters presenting different SOCs can be obtained. This method consumes excessively long time on collecting relevant parameters and corresponding computations are complicated when the issue of the polarization of the battery is taken into consideration. Meanwhile, the equipments implementing such measuring method are expensive and the entire measuring process needs to be accomplished offline, all of which can not fulfill the demands for general battery management systems.